The invention relates to the field of gas turbine engines, and more specifically, to gas turbines maintaining control of fluid density to control system operation and minimize losses.
In conventional gas turbine engines having a turbine and a compressor, turbine output power is controlled by simply varying the fuel supply. When fuel supply is increased, the temperature upstream of the turbine increases, resulting in increased power and speed. This also causes an increase in pressure and in the expansion ratio. Controlling power in conventional gas turbine engines in this way does not pose any significant problems, but these engine are unable to accommodate sudden load changes because the temperature in the gas turbine engine changes over a very wide range: from 600K to 1,400K when going from idling conditions to full load. In addition, it is not possible to xe2x80x9cscale downxe2x80x9d a conventional gas turbine engine to obtain a lower-power, compact engine for uses such as land vehicle applications because the turbine flow duct fluid parameters would require turbine blades to be as small as xe2x85x9 of an inch in height. With such small blades, the engine would not produce enough torque, thus requiring a gearbox and lowering overall efficiency.
These disadvantages can be partly eliminated by reducing the pressure downstream of the turbine with an exhauster. The exhauster allows the expansion ratio to be increased and the pressure upstream of the turbine to be decreased. Turbine blades can then be made larger, and consequently produce more torque than otherwise would have been possible. This does not completely solve the problem because turbine flow duct temperature fluctuations remain. Wide temperature fluctuations result in engine components incurring large thermal expansions and contractions. These deformations result in metal-to-metal clearance variations (which gives rise to losses), lower reliability, and reduced service life.
To mitigate these negative effects, a gas turbine engine can be fitted with compressors on either side of the turbine to control fluid density in the turbine flow duct. The density control range in this case is limited because the pressure would need to be increased by a factor of 100 if power is to be increased from 1 kW to 100 kW. This amount of pressure increase cannot be made rapidly, so the engine response speed will be very slow. To broaden the control range, fluid temperature can be increased as well (from 800K to 1,400K), but as mentioned above, a broad temperature range is undesirable.
Another type of prior art gas turbine engine, which has a compressor, a turbine, a compressor turbine mounted downstream of the turbine for rotation in the opposite direction, and a heat exchanger, has better efficiency because it does not use stator vanes, so losses in the flow duct are lower. If the gas turbine engine of this type has a power output of 50 kW and higher, it can be easily controlled. The temperature in the flow duct in that case varies within the range of 1,250K to 1,400K, providing milder conditions for the gas turbine engine components. If, however, an engine of this type with a power output of say, 25 kW is built with the flow duct dimensions similar to those of a 75 kW engine, flow duct fluid density becomes much lower. This engine has a compression ratio of about 1.0, and it has a narrow control range and a low response speed. If such a gas turbine engine were employed to power a land vehicle, it could not effectively accommodate the sudden load changes that are inherent in this application. Since this type of gas turbine engine is more efficient and can be made more compact to allow it to be used on land vehicles, it is highly desirable to solve this control problem.
It is therefore, an object of the invention to provide a gas turbine engine of the above-described type that can be controlled over the full range of load while maintaining high efficiency.
Another object of the invention is to provide a gas turbine engine that is more reliable in operation.
The foregoing objects are accomplished through the design of a gas turbine engine having a turbine, a compressor turbine mounted downstream of the turbine and which rotates in a direction opposite to the rotation of the turbine. Exhaust fluid from the compressor turbine is cooled in a heat exchanger using a compressed fluid downstream of the compressor and is then cooled with air in a separate heat exchanger before being admitted to the compressor. A part of the compressed fluid heated in the heat exchanger is fed to cool the turbine blades, and the rest of the fluid is fed to a heated fluid source for the turbine. To control the gas turbine engine, the pressure of part of the fluid is increased by a booster compressor and the fluid is discharged from the engine. The booster compressor is driven by an expanding turbine that rotates under the effect of combustion air that is expanded in the expanding turbine. The combustion air flows through the expanding turbine under the action of reduced pressure in the heated fluid source.
Other objects and advantages of the invention will become apparent from the following detailed description of the preferred embodiments thereof and accompanying drawings.